1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to non-volatile magnetic memory.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. For example, after a conventional personal computer is powered off, the volatile memory is typically reloaded through a boot up process upon a restart. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and the like. Disadvantageously, conventional nonvolatile memories are relatively large, slow, and expensive. Further, conventional nonvolatile memories are relatively limited in write cycle capability and typically can only be programmed to store data about 10,000 times in a particular memory location. This prevents a conventional non-volatile memory device, such as a flash memory device, from being used as general purpose memory.
An alternative memory device is known as magnetoresistive random access memory (MRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. Advantageously, MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. There are at least three different types of MRAM devices, including giant magneto-resistance (GMR) MRAM devices, magnetic tunnel junction (MTJ) or tunneling magneto-resistance (TMR) MRAM devices, and pseudo spin valve MRAM devices. GMR MRAM devices separate at least two ferromagnetic layers with a metallic layer. In a MTJ MRAM device, at least two ferromagnetic layers are separated by a thin insulating tunnel barrier. A pseudo spin valve MRAM device uses an asymmetric sandwich of the ferromagnetic layers and metallic layer as a memory cell, and the ferromagnetic layers do not switch at the same time.
One problem that has plagued conventional MRAM devices is relatively low write selectivity. In a conventional MRAM device, the magnetic field applied to a selected memory cell is relatively weakly coupled to the selected cell. This undesirably results in relatively high currents to generate a magnetic field, which is used to write to the cell. The required amounts of current can be higher than desired for relatively high-density integrated circuits. In addition, the magnetic field applied to the selected cell can be undesirably coupled to another cell. This can undesirably cause the contents of an unselected cell to be overwritten.
Embodiments of the invention advantageously solve these and other problems by providing improved write selectivity.